My stepson's psycho ex girlfriend won't leave him alone?

He's 20 years old, old enough to make his own decisions and stuff, I realize that. But this girl is 17 years old, a biker chick, nasty, and just won't leave him alone. They've been on and off again for about a year and a half. None of his family approves, and her mom doesn't like him either and is just as psycho, trying to cause problems with our family. He just gets drunk or lonely or whatever and wants a call and she's the one he calls. Although he used to ignore her numerous calls and texts nearly every day and at one point he wanted her to leave him alone so badly he was going to file a restraining order. I can't stand her and no one else can either. It would be a horrible disaster if she got pregnant or she ended up getting him to marry her or something. What to do?